1. Field of Invention
The present invention relates generally to biased charge rollers for high speed xerographic printing, and more particularly, to biased charge rollers with commutated longitudinal electrodes embedded below the surface of the roller to control the deposition of charge onto a charge retentive surface.
2. Description of Related Art
Typically, electrostatic imaging and printing processes are comprised of several distinct stages. These stages may generally be described as (1) charging, (2) imaging, (3) exposing, (4) developing, (5) transferring, (6) fusing, and (7) cleaning. In the charging stage, a uniform electrical charge is deposited on a charge retentive surface, such as, for example, a surface of a photoreceptor, so as to electrostatically sensitize the surface. Imaging converts the original image into a projected image exposed upon the sensitized photoreceptor surface. An electrostatic latent image is thus recorded on the photoreceptor surface corresponding to the original image. Development of the electrostatic latent image occurs when charged toner particles are brought into contact with this electrostatic latent image. The charged toner particles will be attracted to either the charged or discharged regions of the photoreceptor surface that correspond to the electrostatic latent image, depending on whether a charged area development (CAD) or discharged area development (DAD, more common) is being employed. In the case of a single step transfer process, the photoreceptor surface with the electrostatically attracted toner particles is then brought into contact with an image receiving surface, i.e., paper or other similar substrate. The toner particles are imparted to the image receiving surface by a transferring process wherein an electrostatic field attracts the toner particles towards the image receiving surface, causing the toner particles to adhere to the image receiving surface rather than to the photoreceptor. The toner particles then fuse into the image receiving surface by a process of melting and/or pressing. The process is completed when the remaining toner particles are removed from the photoreceptor surface by a cleaning apparatus.
To charge the surface of a photoreceptor, it is known to use a contact type charging device. The contact type charging device includes a conductive member which is typically supplied a voltage from a power source with a DC voltage (VDC) superimposed with an AC voltage with a peak to peak amplitude (VAC) of at least twice the threshold voltage for air breakdown (VTH). Note that VTH˜312+87.96√{square root over (DOPC)}+6.2DOPC, where DOPC=dOPC/k is the dielectric thickness of the photoreceptor in units of microns (um or μm), dOPC is the thickness of the photoreceptor, and k is the photoreceptor dielectric constant. This equation is valid if the charge roller is sufficiently conductive. For a photoreceptor with DOPC=7.6 microns, VTH=600V and therefore VAC>1200V. When using a conventional DC biased AC BCR (biased charge roller), the photoreceptor charge potential is given by VOPC=VDC, where
      V    OPC    =            σ      ⁢                          ⁢              D        OPC                    ɛ      0      is the photoreceptor charge potential, σ=Q/A is the surface charge density (charge per unit area) deposited on the photoreceptor surface, and ε0 is the permittivity of free space. The charging device contacts the image bearing member (photoreceptor) surface, which is a member to be charged. The outer surface of the photoreceptor is charged by air breakdown in the pre-nip and post-nip air gaps. The contact type charging device charges the photoreceptor to a predetermined potential (VOPC). Typically, the contact type charger is in the form of a roll charger such as that disclosed in U.S. Pat. No. 4,387,980 (see also U.S. Pat. Nos. 4,851,960; 5,164,779; 5,613,173; and 2,912,586) which are hereby incorporated by reference.
In contact type charging systems, it is important that the charging member contacts the charge retentive surface, such as the photoreceptor uniformly along the length thereof. Contact charge type rollers therefore typically include a conformable material to maintain the contact with the photoconductive member. In typical printing applications, AC and/or DC voltages are applied to a roll type charger in contact with a photoconductive drum.
The area between the charge retentive surface and the charge roller surface may be divided into three distinct regions: the nip region, the pre-nip region, and the post-nip region. The nip region comprises the point at which the charge retentive surface and the charge roller surface come into direct contact. The pre-nip region comprises the region upstream from the nip region. In the pre-nip region, there is an air gap between the charge retentive surface and the charge roller surface since the two have not yet come into direct contact. The post-nip region is downstream from the nip region. There is also an air gap between the charge retentive surface and the charge roller surface in the post-nip region.
It is well known that DC biased charging devices have poor charge uniformity because air breakdown in the pre-nip air gap suppresses air breakdown in the post-nip region. It has been demonstrated that post-nip breakdown charges a charge retentive surface, such as, for example, a photoreceptor much more uniformly than pre-nip breakdown. Conventional AC biased charging rollers, a commonly used form of biased charging, use a DC shifted AC high voltage power supply to generate post-nip air breakdown. Although the high voltage AC improves the charge uniformity, it has several disadvantages including additional cost and increased photoreceptor wear.
DC shifted AC biased charge rollers are a part of a commonly used biased charging technology in low volume xerographic engines. Air breakdown occurs in both the pre-nip and the post-nip region. These commonly used biased charge rollers tend to have adequate charge uniformity and they generate little ozone. However, there are several problems associated with the high voltage AC including a high rate of wear, additional costs, banding, and process speed limitations.
For example, the AC biased charge roller deposits both positive and negative charge on the photoreceptor. The positive charge weakens the photoreceptor polymer, which is subsequently abraded by a cleaner blade, increasing wear and reducing its life. It has been shown that eliminating the positive charge deposition from a biased charge roller increases photoreceptor life by a factor of two.
In another example, the conventional biased charge roller further requires an expensive high voltage AC power supply in addition to the DC power supply. The AC also causes audible noise by vibrating the photoreceptor. Noise dampening countermeasures are required to reduce the volume of this noise, adding additional costs.
Further, AC causes high frequency spatial banding (X=VPROCESS/f) that is normally not noticeable to the observer. As the process speed is increased, the frequency of the AC must be increased to prevent the high frequency spatial banding from becoming apparent. The AC current is proportional to 1/f, so a larger, more costly power supply is required.
One of the primary factors limiting the applicability of DC biased charging is non-uniform charging. When sufficiently high negative DC voltage is applied to the shaft of a biased charge roller, the field in the pre-nip regions exceeds the Paschen curve, resulting in air breakdown. Negative charge is deposited on the photoreceptor until the field at all air gaps collapses and lies below the Paschen curve. Therefore there will be no air breakdown in the post-nip regions, and the charging will be non-uniform. However, if the resistivity of the biased charge roller elastomer is precisely tuned so that the charge relaxation time is roughly equal to the dwell time in the nip, the post nip field will exceed the pre-nip field and the charging will be uniform. This is generally not practical because it requires extremely tight control of the resistivity. In addition, when the resistivity is in this “field tailoring” regime, the DC BCR has an increased sensitivity to ghosting defects that result when the charge deposited on the photoreceptor depends on the discharge pattern (latent image) from the previous photoreceptor cycle. This can lead to a “ghost image” of the previous latent image appearing on the print.
The electroded DC biased charging roller described here enables post-nip charging and excellent charge uniformity without suffering from the AC biased charge roller disadvantages described above. The electroded DC BCR also avoids the problems associated with the field tailoring DC BCRs described above.
The present invention provides a biased charge roller wherein a uniform electrical charge is deposited on the surface of a photoreceptor so as to electrostatically sensitize the surface. The biased charge roller of the present invention has several advantages in that the biased charge roller of the present invention provides uniform charging and allows for substantial decreased wear and costs.